The negative voltage-current characteristics of ionized gas lamps such as fluorescent, sodium vapor, neon, and others preclude the direct operation of such devices from D.C. or low frequency A.C. power sources. Once these devices are ionized, the voltage drop of the lamp decreases thereby allowing more current to be drawn. The increase in current further reduces the voltage drop across the lamp--the result being a rapid run away condition which destroys the lamp. Some means of limiting the current is therefore necessary to prevent this condition--while at the same time permitting sufficient current to operate the lamp at its desired operating point, i.e., that current which maximizes the ratio of the lumunous intensity relative to the power input.
Various arrangements of inductors, transformers, and resistors have been used to ballast gaseous discharge lamps. One of the most common devices is an autotransformer which steps up the input voltage and provides sufficient leakage inductance for limiting the current. These devices are heavy, bulky and expensive. In D.C. systems, a resistor is used in series with the lamp--but the power dissipation typically reduces the overall lamp ballast efficiency to approximately 50%.
More recently, high frequency transistor/transformer inverters have been utilized to reduce the physical size of the inductance, or to eliminate it entirely. Most of these ballasts produce a high voltage/high frequency square wave from rectified 60 cycle A.C. Alternatively, the inverter transformer may be eliminated by a voltage doubler and the output voltage therefrom can be chopped to produce the desired high frequency as taught in U.S. Pat. No. 4,117,377 referred to herein above. The generation of high frequencies, however, whether by means of a chopper or inverter, is somewhat costly in terms of the components required, and in the case of the inverter, additional filtering is required to reduce TVI and RFI in most industrial and household applications.
The use of capacitors as ballasts at a first glance appears ideal inasmuch as they dissipate little or no power and are generally smaller and less expensive than inductors and transformers. These characteristics can, in fact, be taken advantage of at high frequencies. Capacitors however, have not proved practical at low frequencies (e.g. 60 cps). A paramount reason for their inapplicability lies in the fact that the lamp must be restarted each half cycle when the AC current reverses. Since the reignition potential exceeds the operating voltage, the ballast capacitor will be abruptly discharged when ignition occurs. For example, if the current resistance is 10 ohms and the reignition potential of the lamp exceeds its operating voltage by 50 volts the discharge waveform of a series connected 10 .mu.f capacitor is a pulse having a duration of 0.1 millisec and an amplitude of 5 amperes. Thus, pulse which occurs 120 times/sec for 60 cps power sources is short compared to the 16 millisec wavelength. As a consequence, the current flows in spikes which have a short duration and high magnitude. The light thus occurs in flashes, and the high current peaks cause rapid deterioration of cathode/filament electrodes. If sufficient series resistance is added to increase the discharge time and "smooth out" the current, considerable power will be dissipated in the resistor, and the overall lighting efficiency will be drastically reduced. What is actually desired is an arrangement which will take advantage of the low loss characteristics of a capacitor but which will permit a fluorescent or other ionized gas lamp to be properly operated from a standard low frequency power source.
Accordingly, a primary object of the present invention is to provide a simple inexpensive low-loss ballast for an ionized gas lamp.
A further object of the invention is to provide a capacitive arrangement for operating an ionized gas lamp from a standard 50 or 60 cps power source.
A further object of the invention is to provide a capacitive arrangement for effectively stepping up the 60 cps line voltage.
A further object of the invention is to provide an apparatus for starting and operating a fluorescent lamp which does not require an inductance to limit the current.
Another object of the invention is to provide a capacitive ballast which will operate from a standard household power source to provide a relatively smooth and continuous lamp current over a period which is equal to or greater than 50% of the input cycle.
Another object of the invention is to provide a low-loss ballast which does not require a chopper or inverter to raise the operating frequency of the voltage applied to the lamp.
A further object of the invention is to provide an arrangement for advancing the reignition point of a capacitor operated ionized gas lamp during each half cycle of the applied A.C. power source.
A further object of the invention is to provide a dual valued capacitance ballast having a low value at the time of ignition and a higher value during the remainder of the conduction phase.
A further object of the invention is to provide a first value of capacitance for reigniting a gas discharge lamp during each half cycle of the power source, and a 2nd value of capacitance for providing a relatively constant current during the conduction portion of each half cycle of the input wave form.
Another object of the invention is to provide an arrangement for ballasting an ionized gas lamp which will provide during each half cycle of the A.C. power source, an "off" interval for charging a striking capacitor, a first "on" state interval for discharging the striking capacitor so as to reduce the lamp voltage and initiate lamp current, and a second "on" state interval for maintaining the lamp current at a value equal to that which is produced by capacitatively differentiating the input voltage source.
Other objects and advantages of the invention will be obvious from the detailed description of a preferred embodiment given herein below.